Transformation collectively refers to those processes in which molecular rearrangements occur, that is, reactions. Once a cell adsorbs to a surface or becomes embedded within an existing biofilm, it will continue its metabolic processes in response to its immediate environment. Three fundamental rate processes can be identified: (1) alginate or exopolymer secretion, (2) cellular growth and replication, and (3) cell death and lysis. Prior to the mid-1990s; it was often easier to measure "per reactor area" performance using observed rates determined from bulk liquid parameters (e.g., electron donor removal rate or electron acceptor uptake rate) than to directly measure growth, replication, and death of cells in a biofilm. This experimental limitation led to the development of a number of unstructured mathematical models that were useful in estimating the flux of growth-rate-limiting substrate into a biofilm of fixed thickness, density, and reactivity, taking into account both external and internal molecular diffusion and biological reaction. It was not until the advent of various molecular methods and advances in computer-aided microscopy that it was possible to determine various biofilm processes directly within the biofilm.
Adhesion-Induced Alginate Synthesis. Reporter gene technology has been used to observe the regulation of the alginate biosynthesis gene, algC, in a mucoid strain of P. aeruginosa in developing and mature biofilms (58). In vivo detection of algC up-expression in developing biofilms was performed with a fluorogenic substrate for the plasmid-borne lacZ gene product (S-galactosidase) using microscopy coupled with image analysis. By this technique, cells were tracked over time and analyzed for algC activity. During the initial stages of biofilm development, cells attached to a glass surface for at least 15 min exhibited up-expression of algC, detectable as the development of whole-cell fluorescence. However, initial cell attachment to the substratum appeared to be independent of algC promoter activity. Furthermore, cells not exhibiting algC up-expression were shown to be less capable of remaining at a glass surface under flowing conditions than were cells in which algC up-expression was detected.
Cell Growth and Replication. The major transformation carried out by cells in the biofilm is the metabolism of both an electron donor and a terminal electron acceptor to produce soluble by-products, extracellular polymers, carbon dioxide, and water. Depending on the microbial population in question and the ambient concentration of electron donor and acceptor, a biofilm can be either aerobic, anoxic (denitrifying), anaerobic (sulfate-reducing bacteria, methane formers), or fermentative. Analysis of biofilm bacterial metabolic rates are frequently complicated by the effects of significant mass transfer resistances in both the liquid phase and within the developing biofilm. steady-state mathematical models (59,60) were derived in the late 1970s to estimate the observed flux of growth-rate-limiting substrate into a biofilm of fixed thickness, density, and reactivity. These steady-state biofilm models are based on assumptions of a constant biofilm concentration (tacitly implying a spatially uniform reactivity) and a constant diffusion path (biofilm thickness). such models allow one to predict (1) the concentration profile of limiting substrate (and, by stoichiometry, all other nutrients) with biofilm depth, and (2) the maximum substrate uptake or flux to the biofilm.
Biofilm cellular growth rates are typically simulated using a classical microbial growth rate expression, Rg, that is related by a stoichiometric coefficient, Y (in units of cell mass per substrate mass), to the rate of utilization of growth-limiting substrate, RS (Msubstrate L~2f_1), that is,
where the rate of substrate utilization is typically a Monod-like dependency on the concentration of one or more growth-rate-limiting substrates. For the example of a single substrate limitation,
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